The present invention relates to novel polyurethane compositions which are advantageously suitable for the production of holographic media for purposes including data storage, but also for optical applications of a wide variety of types.
Holographic media may be used inter alia for data storage, in the field of security technology (for example the three-dimensional representation of people or objects and the authentication of a person or of an article), the representation of objects, for advertising, as auxiliary materials for production of complex three-dimensional structures and as a visual display unit or as components of visual display unit parts, and for production of an optical element with the function of a lens, of a mirror, of a filter, of a diffusion disc, of a diffraction element, of a light conductor and/or of a mask.
Holographic media based on a polymer matrix and one or more free-radically polymerizable compounds embedded therein writing monomers are described, for example, in U.S. Pat. No. 6,743,552.
U.S. Pat. No. 6,743,552, U.S. Pat. No. 6,765,061 and U.S. Pat. No. 6,780,546 disclose two-component polyurethane formulations which, however, do not comprise any prepolymers having primary NCO groups.
U.S. Pat. No. 6,743,552, U.S. Pat. No. 6,765,061, U.S. Pat. No. 6,780,546 and U.S. 2006/0194120 disclose two-component polyurethane formulations. Some of the formulations described therein contain prepolymers as the isocyanate component, these being exclusively those prepolymers with secondary isocyanate groups, and therefore being unsatisfactory in terms of curing rate.
JP 2007101743 describes the use of prepolymers consisting of dicyclohexylmethane 4,4′-diisocyanates (“H12-MDI”) and trifunctional polypropylene oxide glycerol ether with addition of 1,4-butanediol as a chain extender for the production of PU matrices for holographic media for optical data storage.
However, a disadvantage of the known systems based on polyurethane, especially for optical applications outside digital data storage, is that the achievable brightness of the holograms stored in such media is too low. The reason for this is usually that the relative difference in the refractive indices of polyurethane matrix and writing monomer is too small. An arbitrary variation in the matrix polymer, though, is impossible since a good compatibility of matrix polymer with writing monomer and the further components present in the formulations must always be ensured. Moreover, for processing reasons, the simplest possible mixing and provision of the formulations is of interest.
It was therefore an object of the present invention to provide novel polyurethane compositions which meet the abovementioned requirements.